Lift trucks are used extensively in storage areas such as warehouses. Goods are mounted on a pallets having a platform that is spaced from the floor a distance sufficient to introduce the forks to the lift truck beneath the platform. The pallets are adapted to be lifted and moved to locations where the goods are to be deposited.
In a typical warehouse where the goods are stored on shelves, the fork raises the goods to the "store height" of a selected shelf to deposit the goods on the shelf. The store height is the height to which the pallet is raised so the pallet can be moved over and lowered onto the shelf. To retrieve a pallet from a shelf, the fork is raised to a "retrieve height" of the selected shelf, the height at which the fork enters between the platform of the pallet and the shelf. The fork is inserted under the platform of the pallet, is raised to lift the pallet off the shelf, and is withdrawn from the shelf with the pallet resting on the fork. The store height is higher than the retrieve height by an amount sufficient to provide adequate clearance between the pallet and shelf as the pallet is moved into the storage position over the shelf. Four inches is a typical clearance.
Without a device for indicating the height of the fork relative to a selected shelf, the operator of the lift truck must visually determine when the fork is at the correct height relative to a selected shelf to store a pallet on or retrieve it from the shelf. This is very difficult because of the relatively small difference between the store and retrieve heights, the difficulty in distinguishing the distances involved with high shelves, and obstructions in the field of vision such as the mast of the truck or the load. Also, the consequences of misjudging the height can be severe. Where the fork is too high or too low relative to the retrieve position it may engage the goods or the shelf rather than the pallet. The fork may damage the goods, the shelf, or the pallet, or it may push the pallet and goods off the opposite side of the shelf. Where the fork is too high or too low relative to the store height, the goods on the pallet may be jammed into the shelf and the top of the goods may hit the shelf above the selected shelf. This may damage the goods or the shelf, or the goods may fall off the pallet onto the floor, the truck, or the operator.
To assist the operator in locating the fork accurately relative to the height of the shelf, systems have been proposed to provide the operator with an indicator showing the height of the fork relative to the shelf. The shelf height indicator also permits the operator to raise or lower the fork as the truck is being moved from one location to another so that the truck arrives at the shelf where the goods are to be picked up or deposited with the fork at the correct height. This reduces the time required to raise or lower the fork when the truck is at the desired shelf. The indicator also can be used as a safety device to warn the operator when the fork is too high and the fork or the goods thereon may strike equipment such as air ducts, water lines, or lights on the ceiling of the warehouse. Devices which assist the operator in locating the fork accurately with respect to each shelf must evaluate criteria in addition to the different store and retrieve heights of the shelves. The warehouse may be designed to store various different goods on unequally spaced shelves. The warehouse or warehouse complex also may have different storage areas with shelves that are spaced differently.
U.S. Pat. No. 4,122,957 to Allen et al. discloses a fork lift which senses reflectors 25 opposite each shelf. The sensors actuate a drive for potentiometer 32 on the fork lift. This actuates meter 36 to indicate to the operator the retrieve and store heights for each shelf. The shelf number may be tracked by tape 29 that is wound on reel 30 while the position of the fork relative to the selected shelf is determined by the position of the reflector. However, the reflectors must be located at each storage position along each shelf. This requires the reflectors to be continuous along the shelf if the number of storage positions is not restricted. Continuous reflectors are expensive to install and also require extensive maintenance because their location on the shelf faces exposes them to damage. Also, separate means for indicating the shelf number and the store and retrieve heights for each shelf represents added expense.
The patent to Scholl U.S. Pat. No. 4,074,794 discloses another shelf height determining device for a fork lift truck. In Scholl, an indicator for the operator determines the height of the fork relative to the shelf on an automatic system for controlling the operation of the fork to predetermined heights. Because the operator must read the height from a scale and correlate this height to the store or retrieve heights of a particular shelf, error is likely. Also, the system itself is automatic; the lift is not within the control of the operator. This is expensive and, in the event of a malfunction, often results in damages exceeding that in a system in which the operator is in control and can sense trouble and limit damage.
Ulinski, U.S. Pat. No. 3,542,161 discloses a fork lift height control in which the height of the fork is determined by a cable attached at one end to the fork or load carrying element and at the other end to an indicator. The Ulinski device requires the operator to read and interpret a specific height relative to the desired shelf height. With an automatic system, this is relatively expensive.
The Nakada patent, U.S. Pat. No. 4,411,582 shows a fork lift truck in which both the elevation and tilt of the fork are programmable and are controlled automatically.
Commercially available controls for fork lift trucks include, for example, a unit marketed by Toledo Instrument Company. This device uses a tilt indiCator for the mast which, through a series of lights, indicates when the mast is vertical, is tilted forward, or is tilted backward. A device marketed by Marco Engineering Inc. provides a method for programming the shelf height in a warehouse using a system that automatically controls the raising and lowering of the fork to the selected height.